Transistor signal amplifier circuits



R. o. LOHMAN 2,851,542

TRANSISTOR SIGNAL AMPLIFIER cmcurrs Original Filed llay 28, 1953 Sept.9, 1958 IN VEN TOR. RuBERT D. Lnumn WWW ATTORNEY United States PatentRobert D. Lohman, Princeton, N. J., assignorto Radio Corporation ofAmerica, a corporation of Delaware Continuation of application SerialNo. 357,954, May 28,

1953. This application May 17, 1956, Serial No. 585,521 1 6 Claims. (Cl.179-471) This application is a continuation of my copending applicationSerial No. 357.954, filed on May 28, 1953, now abandoned, entitledReduction of Distortion in Semi- Conductor Amplifier Circuits."

This invention relates generally to signal amplifier circuits having twosignal paths arranged for push-pull operation and particularly tosemi-conductor signal amplifier circuits of that type.

Two classes of semi-conductor devices have been utilized in signalamplifier circuits to which the present invention pertains. One class isa junction transistor which comprises a semi-conductive body having twoend zones of one type of semi-conductive material separated by andcontiguous with an intermediate zone of opposite type of semi-conductivematerial. Electrodes are placed in essentially low resistancc contactwith the respective zones to provide means for external circuitconnections. The electrodes which are in contact with the respective endzones are termed the emitter electrode and the collector electroderespectively. The electrode which is in contact with the intermediate orcenter zone is termed the base electrode. The junction transistor canthen. of course, be of the NP-N type or the P-NP type. These two typesof junction transistors will hereinafter be referred to as beingopposite conductivity types.

The second class of semi-conductor devices is known as the point contacttransistor which comprises a semiconductive body having two electrodesin high-resistance or rectifying contact therewith and a third electrodein low-resistance contact therewith. The semi-conductive body may be agermanium or silicon crystal of the N or P type. The two electrodeswhich are in high-resistance contact with the semi-conductive body aretermed the emitter electrode and the collector electrode. The electrodewhich is in low-resistance contact with the semiconductive body istermed the base electrode. As mentioned above in connection with thejunction transistor, point contact transistors oi the N and P type willalso be referred to hereinafter as opposite conductivity types. It is,however, noted that an N type point contact tran sistor is of the sameconductivity type as a P-N-P junc tion transistor.

Transistors have been applied to signal amplifier circuits havingparallel signal paths to provide push-pull operation. It has been found,however, that these circuits may require adjustment upon substitution oftransistors due to the fact that each of the signal paths should haveidentical characteristics statically and dynamically in order to providestability in operation and substantially distorionless signal output. Ithas been found, however, that even though transistors are manufacturedwith an attempt to provide uniformity of characteristics, the ultimatechar acteristics of the units may vary within wide limits. Thisvariation of transistor characteristics may cause a circuit which hasbeen properly adjusted for one set of transistors to operateunsatisfactorily with a substitute set of transistors. It has also beenfound that due to the fact that there are parallel signal paths, thereis a required sym- Patented Sept. 9, 1958 metry of operation betweenthe'two signal paths. Consequently, a difference of transistorcharacteristics between the transistor or transistors utilized in one ofthe signal paths'and the characteristics of the transistor ortransistors utilized in the other signal path may result in unstableoperation and distortion.

Accordingly. it is an object of the present invention to provide animproved substantially distortionless semiconductor amplifier circuit ofthe push-pull type.

It is another object of the present invention to provide an improvedclass B push-pull. semi-conductor amplifier circuit which issubstantially free from crossover distortion.

It is still another object of the present invention to provide animproved semi-conductor amplifier circuit which enables stable push-pullclass B operation with semi-conductor devices having a wide variety ofoperating characteristics while introducing substantially no crossoverdistortion.

In accordance with one embodiment of the present invention, a pair ofsemi-conductor devices or transistors of opposite conductivity type areconnected in parallel between an amplifier input and output circuitterminals, that is, corresponding input electrodes of the transistorsare connected to a terminal of the input circuit and cor respondingoutput electrodes of the transistors are connected to a terminal of theoutput circuit. One electrode of each transistor is common to the inputand output circuits. Appropriate bias voltages are applied as describedmore fully herein. Crossover distortion is prevented, in general, byconnecting an impedance element in the input circuit between the inputelectrodes of the pair of transistors. Accordingly, a bias voltage isprovided such that the input electrodes may both be slightly biased inthe forward direction. Since the characteristics of the two signal pathsare maintained substantially equal but of opposite conductivity type. asingle-ended input signal results in push-pull amplification withoutdistortion.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in which:

Figure l is a schematic circuit diagram of a semi-conductor amplifiercircuit embodying the present invention;

Figure 2 is a schematic circuit diagram of a semi-conductor amplifiercircuit illustrating a further embodiment of the present invention; and

Figure 3 is a schematic circuit diagram of a semi-conductor amplifiercircuit in accordance with with present invention and illustrating amodification of the schematic circuit diagram shown in Figure 2.

Referring now to the drawing wherein like elements have been designatedby the same reference characters throughout the various figures. andparticularly to Figure 1 four transistors 10, ll, 12 and 13. which maybe junction transistors, are connected in a parallel path signalamplifier circuit. Input signals for the driving amplifier may beappliedto an input circuit comprising a pair of input terminals 14 andan input resistor 15. The high voltage terminal of the input terminals14 is coupled to each of the base electrodes 16 and 17 by means of apair of equalizing resistors 18 and 19 as provided in accordance withthe present invention. The other terminal of the input terminals 14 maybe con nected to a point of fixed reference potential such as ground.The emitter electrodes 20 and 21 of the respective drivingtransistorsll) and 11 are connected in common to the high voltageterminal of an output impedance illustrated as an inductor 22, whichcould be the voice coil of a loudspeaker, for example, which hasresistance. It is, of course, to be underst od that this outputimpedance may take substantially any form, and may be a resistor, forexample. In order to provide driving currents from the outputtransistors 12 and 13, the collector electrodes 24 and 25 of therespective driving transistors 10 and 11 are directly connected to thebase electrodes 26 and 27 of the respective output transistors 12 and13.

Bias voltages for the circuit are provided by a pair of batteries 28 and29 which are respectively connected between the emitter electrodes 30and 31 and a point of fixed reference potential such as ground. Thebatteries 28 and 29 may be bypassed for alternating current frequenciesby a pair of capacitors 32 and 33. The circuit of the output transistors12 and 13 is completed by connecting their respective collectorelectrodes 34 and 35 to the high voltage terminal of the load impedanceelement 22.

While it will be understood that the circuit specifications may varyaccording to the design for any par ticular application the followingcircuit specifications are included by way of example only.

Transistor l and 13 2N34 Transistor l0 and 11 2N35 Resistor 18 and 19ohms 3,000 Battery 28 and 29 "volts-.. 7.5 Voice coil 22 ohms 16 As tothe operation of the circuit illustrated in Figure 1, let it first beassumed that the equalizing resistors 18 and 19, provided in accordancewith the present invention, have been omitted and that the circuit hasbeen adjusted for class B operation. Under these conditions, it isreadily seen that if the characteristics of the transistors and 12 inone of the parallel paths differs from the characteristics of thetransistors 11 and 13 in the other of the parallel paths, a differentialcurrent will flow through the load impedance element 22 developing adirect current voltage thereacross which will be a static bias appliedto the emitter electrodes 20 and 21. It is also to be noted that aleakage current due to the leakage across the collector junction in eachof the two transistors 10 and 11 may be flowing in the base electrodes16 and 17. This leakage current is in a direction opposite to the normalbase current which is produced by forward bias between the base and itsrespective emitter electrode.

Accordingly a current in the base electrode 16 produced by this leakagewill be flowing out of the base electrode 16 and the current flowing inthe base electrode 17 produced by the leakage current will be fiowinginto the base electrode 17. The resulting static bias appearing at thebase electrodes 16 and 17 will then be the voltage drop appearing acrossthe input resistor which is produced by the algebraic sum of the basecurrents flowing therethrough.

Under these conditions let it be assumed that the unbalance in the twoparallel paths is such as to produce a differential current through theload impedance 22 in such a direction as to develop a negative voltageacross the load impedance 22 with respect to ground. Accordingly, aforward bias will appear between the base electrode l6 and itscorresponding emitter electrode thereby reducing the base current.However, the reverse bias appearing between the base electrode 17 andits corresponding cmittcr electrode 21 is maintained since it isimpossible to have both transistors biased in the forward direction iftheir base electrodes and emitter electrodes are connected together.

The result of this forward bias applied between the base and emitterelectrodes of the transistor 10 is to increase the currcnt of the outputtransistor 12 and thus decrease the differential current flowing throughthe output impedance 22. However, to accomplish this result it isnecessary that the bias between'the base electrode 16 and the emitterelectrode 20 remain ina forward direction, and, therefore, the emitterelectrode 20 must remain slightly negative with respect to itscorresponding base electrode 16. It is thus readily seen that withoutthe benefit of the present invention unequal bias conditions may existas to the two parallel signal paths, and accordingly crossoverdistortion will result when an alternating current signal is appliedthereto.

- Now let it be assumed that two tequalizing resistors 18 and 19 havebeen added and the circuit. as in accordance with the present inventionas illustrated in Figure l. The effect of these two equalizing resistorsin combination with the leakage current above referred to is to providea small amount of forward bias on both of the transistors 10 and 11.That is to say, the leakage current flow in the transistor 10 is out ofthe base 16 which will tend to forward bias the emitter-base diode ofthe transistor 10 due to the voltage drop across the resistor 18. Theleakage flow in the transistor 11, on the other hand, is into the base17 which will tend to forward bias the emitter-base diode of thetransistor 11 due to the voltage drop across the resistor 19. Thus thevoltage between the emitter elcctrodcs and their respective baseelectrodes is such as to make each emitter capable of emitting minoritycarriers. The equilibrium condition described previously does not nowcut off or apply a further reverse bias between the base electrode 17and the emitter electrode 21 since it is possible for each of the baseelectrodes to be at different potentials. Therefore, both units may bebiased in a forward direction even though each emitter electrode isconnected to a common point.

When a positive pulse of input signal is applied to the input electrodes14, the current flowing in the base electrode 16 will increase slightlyand saturate at the leakage value. The leakage current in the baseelectrode 17 will decrease, pass through zero, and reverse as the signalbecomes more positive. However, it is seen that the crossover is smoothand not notched as there is no time during which the emitter electrodesare not in condition to emit minority carriers.

The schematic circuit diagram shown in Figure 2 illustrates a furthermodification of the present invention as applied to a single pair ofjunction transistors 43 and 44. Accordingly. an impedance elementillustrated as a resistor 40 is connected between the base electrodes 41and 42 of a pair of semi-conductor devices 43 and 44 which are arrangedas a parallel path signal amplifier, better known as a complementarysymmetry class B pushpull amplifier. Bias voltages for this circuit areprovided by a pair of voltage sources illustrated as batteries 45 and 46connected respectively between the collector electrodes 47 and 48 andground. The batteries 45 and 46 may be bypassed for alternating currentsignals by a pair of capacitors 49 and 50. A load impedance element 39illustrated as a rectangle containing the legend 2;, is connectedbetween a point of fixed reference potential such as chassis ground andthe two emitter elec trodes 51 and 52.

Static bias is provided for the base electrodes 41 and 42 by means of abias or voltage divider network comprising two rcsistors 53 and 54connected in series with the resistor 40 between the collectorelectrodes 47 and 48. The base electrode 41 is connected to the junctionof the voltage dividing resistor 53 and the resistor 40 and the baseelectrode 42 is connected to the junction of the voltage dividingresistor 54 and the equalizing resistor 40 of relatively low resistance,for example approximately 30 ohms. An input signal may be appliedsimultaneously to the base electrodes 41. and 42 in substantially thesame phase and amplitude by means of a pair of input terminals 55. oneof which is connected to signal current ground and the other of which isconnected to the base electrode 42 through a coupling capacitor 56.

A difference in the operating characteristics of the two transistors 43and 44 may result in a ditferential current flowing through the lotlimpedance 39 which will, in turn, cause a static bias voltage to bedeveloped thereacross. This static bias voltage will be simultaneouslyapplied between the emitter electrodes 51 and 52 and the correspondingbase electrodes 41 and 42. With the cir cuit adjusted for class Boperation, leakage currents in the two transistors 44 and 43 may resultin reverse base current flowing in each of the base electrodes 41 and42. Accordingly the leakage current flowing in the base electrode 41will be flowing into the base electrode 41 and the leakage currentflowing in the base electrode 42 will be flowing out of the baseelectrode 42.

Under these circumstances it is readily seen as above discussed that thestatic bias which is developed by the diiferential current across theload impedance 39 is such as to oppose the effect of the current in oneof the two transistors but the eflect of the leakage current in theother of the two transistors remains unaffected. This, of course, willresult in crossover distortion when an alterhating current; signal isapplied to the input terminals 55.,

However, the addition of the bias network comprising the voltagedividing resistors 53 and 54 and the equalizing resistor 40 provides avoltage bias between the bases 41 and 42 to overcome this difliculty.

The values of the voltage dividing resistors 53 and 54 and theequalizing resistor 40 are chosen initially to provide a proper biasbetween the base electrode 41 and its corresponding emitter electrode 51and between the base electrode 42 and its corresponding emitterelectrode 52. For class B operation these bias voltages will be small. Asmall voltage dillerence exists between the tWO base electrodes 41 and42 due to the voltage drop across the equalizing resistor 4!) ofrelatively low resistance value. Accordingly, the divider networkcomprising the two voltage dividing resistors 53 and 54 and theequalizing resistor 40 provides a slightly forward bias between each ofthe base electrodes 41 and 42 and their corresponding emittcr electrodes51 and 52.

If, as would be conventional. the equalizing resistor 40 were omittedfrom the circuit, one of the transistors would ordinarily be reversebiased. However, it is readily seen that the voltage difference existingbetween the base electrodes; 41 and 42 prevents such a condition andforward biases the emitters both of transistors rela tive to theirrespective oases thereby providing operation without crossoverdistortion. In accordance with the present invention. it is ptelcrredthat the resistance of the compensating element 40 be low compared tothe input impedance of the transistors and that the current which flowsin the resistive network 53, 40, 54 with the transistors disconnected belarge compared to any expected base leakage currents. Because of the lowresistance path connecting the base electrodes 41 and 42, the nal isapplied in substantially the same phase and amplitude to each of thebase electrodes.

Figure 3 illustrates the application of the present invention to a pairof scmi-cnduct0r devices 43 and 44 arranged in a parallel path signalamplifier circuit substantially identical with that shown in Figure 2.However. in this instance a driving transistor 60 is utilized as thesignal source for the parallel path signal amplifier and accordingly thecollector electrode 61 is connected directly to the base electrode 42.

An emitter impedance element illustrated as a resistor 62 is connectedbetween the emitter electrode 63 and the positive terminal of thebattery 46. Proper biasing voltage for the base electrode 64 is providedby a pair of bias resistors 65 and 66 which are connected in seriesarrangement between the negative terminal of the battery 45 and thepositive terminal of the battery 46. The base electrode 64 is connecteddirectly to the junction of these two bias resistors 65 and 66. An inputsignal may be input sig-v applied to the transistor 60by means of acoupling capacitor 67 which is connected between the base electrode 64and one of a pair of input terminals 68, the other of the pair of inputterminals 68 may be connected directly to signal ground.

In operation this circuit is substantially identical with the operationof the circuit above described in connection with Figure 2. It may benoted that in this instance pensating action as provided by the resistor40 as part of the voltage divider network is substantially identical tothat above noted and the emitter-base circuits of each of the outputtransistors will be biased in the forward direction.

It will be appreciated that while this invention has been described byreference to point contact and junction transistors that it is in no waylimited to transistors of these specific forms. Other types which caloperating characteristics in the N and P forms may also be used in thepractice of this invention even though they may differ in a detailedmanner by which the output of providing push-pull output with a minimumof distor tion. It is to be understood that specific embodiments of theinvention shown and described are illustrative and that variousmodifications may be made therein without departing from the scope andspirit of this inventionv What is claimed is: 1. In a push-pull class Bsignal amplifier circuit, the

I combination comprising. a first transistor of one conter electrodesfor developing a push-pull output signal; means providing adirect-current supply source for said circuit connected with said firstand second collector electrodes for applying biasing potentials thereto;and means providing a voltage divider network including first impedancemeans connected between said first base electrode and said source.second impedance means connected between said second base electrode andsaid source, and a resistor having relatively low resistance connectedbetween said first and second base electrodes; said voltage dividernetwork providing forward bias between said first emittcr and first baseelectrodes and between said second emitter and second base electrodes toreduce crossover distortion in said amplifier circuit.

2. In a push-pull class B signal amplifier circuit the base electrodes,and impedance means connected between said second base electrode andsaid source; said voltage divider network providing forward bias betweensaid first emitter and first base electrodes and between said secondemitter and second base electro ies to reduce crossover distortion insaid amplifier ClfCLlil.

3. A push-pull amplifier circuit as defined in claim 2 wherein saidimpedance means comprises a third resistor.

4. A push-pull amplifier circuit as defined in claim 2 wherein saidimpedance means comprises a third transistor.

5. A push-pull class B signal amplifier circuit comprising, incombination, a first transistor of one conductivity type including afirst base, a first emitter, and a first collector electrode; a secondtransistor of an opposite conductivity type including a second base, asecond emitter, and a second collector electrode; means directly conmeeting said first emitter electrode with said second emitter electrode;load impedance means connected with said first and second emitterelectrodes for developing a pushpull output signal; means providing adirect-current supply source for said circuit connected with said firstand second collector electrodes for applying biasing potentials thereto;and means providing a voltage divider network including a first resistorconnected between said first base electrode and said source, a secondresistor having relatively low resistance connected between said firstand second base electrodes, and a third transistor connected betweensaid second base electrode and said source; said third transistorincluding a third collector electrode direct-current conductivelyconnected with said second base electrode and a third emitter electrodeconnected with said source; said voltage divider network providingforward bias between said first emitter and first base electrodes andbetween said second emitter electrodes and second base electrodes toreduce crossover distortion in said amplifier circuit.

6, A push-pull class B signal amplifier circuit comprising, incombination, a first transistor of one conductivity type including afirst base, a first emitter, and a first collector electrode; a secondtransistor of an opposite conductivity type including a second base, asecond emitter, and a second collector electrode; means directlyconnecting said first emitter electrode with said second emitterelectrode; load impedance means connected with said first and secondemitter elecrodes for developing a pushpull output signal; meansproviding a direct-current supply source for said circuit connected withsaid first and second collector electrodes for applying biasingpotentials thereto; a first resistor connected between said first baseelectrode and said source; a second resistor having relatively lowresistance connected between said first and second base electrodes andproviding a direct-current conductive path therebetween; a thirdtransistor of said one conductivity type including a third base, a thirdemitter, and a third collector electrode; input circuit means connectedfor applying an input signal between said third base and third emitterelectrodes; means direct-current conductively connecting said thirdcollector electrode with said second base electrode; and meansconnecting said third emitter and third base electrodes with saidsource; said first and second resistors and said third transistorcomprising a voltage dividing network for forward biasing said firstemitter and first base electrodes and said second emitter and secondbase electrodes to reduce crossover distortion in said amplifiercircuit.

References Cited in the file of this patent UNITED STATES PATENTS2,217,269 Foster Oct. 8, 1940 2,647,958 Barney Aug. 4, 1953 2,651,685Tharp Sept. 8, 1953 2,652,460 Wallace Sept. 15, 1953 2,666,818 ShockleyJan. 19, 1954 2,666,819 Raisbeck Jan. 19, 1954 FOREIGN PATENTS 665,867Great Britain Jan. 30, 1952

